Aircraft consist of many parts that are assembled using fasteners. Certain fasteners, such as those within fuel tanks, must be protected from fuel leakage. For other fasteners, such as those on the exterior surface of an aircraft, it is desirable to smooth depressions such as those resulting from countersinks and panel gaps to improve aerodynamic performance and to provide electrical insulation. Compositions used to seal these and other fasteners must meet the demanding aerospace requirements. Furthermore, because of the large number of fasteners and panel gaps on an aircraft, which can be several thousand, it is desirable that the sealant compositions used and the methods of applying the sealant composition be amenable to efficient high-volume processing.
Preformed seal caps having a partially hardened shell comprising a first sealant filled with a partially unhardened second sealant have been used to seal fasteners. These seal caps are maintained at a temperature below the hardening temperature of the second sealant until immediately prior to use at which time the seal caps can be placed on a fastener and cured at ambient temperature. Seal caps and methods of forming such seal caps are disclosed in U.S. Pat. No. 7,438,974. The premixed and frozen seal caps described in U.S. Pat. No. 7,438,974 cure within about two days and are opaque due to the nature of the thermally-curable polymers employed. However, it is desirable to provide a preformed seal cap having a faster curing time and that is visually transparent. A visually transparent sealant material, both before and after curing, is desirable in order to facilitate visual assessment of the quality of the seal between the sealant and the component such as a fastener. For example, voids or bubbles observed at the interface between the sealant and the part that become apparent after sealing or during use can indicate a need for repair or replacement.
There is a need, therefore, for aerospace sealants for use with fasteners and panel gaps that cure rapidly, that are amenable to visual inspection, and that meet the performance requirements for aerospace sealants. There is also a need to smooth and electrically insulate defects in aerospace surfaces that is amenable to high volume production.